Correct Beta anchor on the link

What does this MR do and why?

Updates the link to "Beta" when the duo_chat_beta flag is enabled.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Set up Duo Chat
• Enable the duo_chat_beta flag (in your rails console: Feature.enable(:duo_chat_beta))
• Open chat (Help - GitLab Duo Chat)
• In the chat's header, note the Beta button. Click it
• The anchor in the text saying "Beta" should point to "Beta" documentation and not "Experiment"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.